A term “sliver” is generally understood to mean a tow-form mass of fibers which is composed of discrete unit fibers that are aligned in parallel to form a bundle without causing unnecessary entanglement and has a length that can be regarded as infinitely long compared with those of the unit fibers. (A part of such a concept is found in an upper left column on page 2 of Patent document 3 shown below.) Carbon fiber sliver is useful as a half product for various carbon fiber products. More specifically, a carbon fiber sliver may be processed by spinning to provide a spun yarn, and such a spun yarn may be woven to provide a carbon fiber textile (cloth). Further, a carbon fiber sliver may be milled or pulverized to provide a milled product or cut to a length of 100 mm or below to provide chops; and chops may be transformed into a paper through a wet process or into a mat through a dry process, or cut, disentangled, laid in layers and needle-punched to provide a felt, respectively. These carbon fiber products are widely used as products, such as heat-resistant materials, electroconductive materials, reinforcing materials and thermally insulating materials by utilizing the properties thereof, such as heat resistance, electroconductivity and strength.
As for processes for producing carbon fiber slivers, Patent document 1 shown below discloses a process for producing a carbon fiber spun yarn wherein a carbon fiber precursor sliver having fiber lengths of at least 25 mm, preferably 50-75 mm is spun, as it is or after it is made preliminarily fire-resistant according to necessity, and heated above its carbonizing temperature. However, the lengths of fibers in the sliver used in the process are short, and the resultant strength cannot be regarded as sufficient. Patent document 2 shown below discloses a process wherein a pitch-based carbon fiber is mixed with a natural fiber and/or a synthetic fiber and disentangled to provide mixed fibers which are carded into a fleece and then into a sliver, and the sliver is drawn and twisted simultaneously to provide a spun yarn. However, this process requires a troublesome step of further heat-treating the natural fiber and/or synthetic fiber for conversion into carbon fiber and also involves a problem that physical property changes caused by, e.g. shrinkage due to carbonization, have to be predicted in advance. Further, it is impossible to obtain a spun yarn of sufficient tensile strength because of shortness of the used fibers.
Patent document 3 shown below discloses a process for producing a carbon fiber sliver wherein various forms of pitch-based carbon fiber mass obtained after spinning and calcination are mixed with 10-40 wt. % of carbon precursor fibers other than pitch-based one to provide a mixture fleece, and the mixture fleece is subjected to drawing and doubling after carding or directly without carding, followed by carbonization of the resultant doubled fiber to provide a sliver principally comprising pitch-based carbon fiber. In the production process, the mixing of the pitch-based carbon fiber and the carbon precursor fiber may be performed by using a generally well-adopted air blowing disentanglement and fleece-forming method but sufficient disentanglement and disintegration of starting fibers are required in order to accomplish a uniform mixing in the air blowing disentanglement step, so that the process involves a problem of requiring preliminary cutting of the pitch-based carbon fiber and the carbon precursor fiber into short fibers of 5-30 mm. Further, it is presumed impossible to obtain a strong spun yarn because of the short fiber length.
Patent document 1: JP-A 53-81735,
Patent document 2: JP-A 8-158170,
Patent document 3: JP-A 1-148813,
Patent document 4: JP-A 62-33823, and
Patent document 5: JP-A 50-6822.